Researchers in Oxford have discovered that the HIV virus in some parts of the world may be weakening as the human immune system evolves to battle it.

Philip Goulder of the University of Oxford told the AIDS Vaccine conference that these changes seemed to be happening surprisingly fast, and that in some populations the introduction of antiretroviral therapy would also reduce the fitness of the HIV that was still circulating.

The immune system works by looking at the proteins on cells. When the body’s HLA genes see a protein they don’t recognise, they will latch on and attack it. HIV works by stopping this latching-on process – but some people in South Africa and Japan seem to have a mutated HIV protein which is actually recognised quicker and fought more strongly by the body.

Three-quarters of people with HIV carry this mutation in Japan, versus less than half in Oxford. In Durban, meanwhile, there appears to be a high proportion of women who are controlling HIV spontaneously. 10% of untreated women had a viral load under 50, compared to no more than 0.5% in other places.

But that’s no reason to stop getting check ups and going for treatment – because, as the researchers point out, those people who have strong HIV in their system are likely to die sooner, making them less able to pass it on. And in populations like the UK, where most people diagnosed are treated, the majority of HIV will be passed on by the undiagnosed.